A good idea is to start with PC magazines, and visit some PC sites on the web that has good advice, try something like tomshardware.com, they provide good advice, benchmarks, the lot.
Always buy components made by reputable brands that have been around, like asus, gigabyte and the many there are.
But be aware, there are some funny chinese jobbies flying around that is not good.
If you really want to get your hands dirty see what chipset are on the mobo for example and see how they compare to others. Good sites/magazines will always report if they've had trouble with a given chipset. Remember these guys make their living off computers, they test just about everything
That approach goes also with memory, graphics cards, PSU's, etc. Seagate barracuda rules the roost when talking hard drive at this point.
It's also rather easy to put everything together these days too. The hardware manuals have been improved a lot, especially the established brands, far cry from the japanese-english it used to be.
I tend to replace my PC around every three to four years, mainly for performance reasons, and still uses the old one afterwords for a very long time.
Although at this point I have such a lot, all functional, but in storage, and a bit slow.